I found a patent which was granted but the product which is claiming the patent is different from the contents of the patent.
How does one challenge the validity of the patent?
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A patent can be challenged in an ex party re-examination or an inter partes review at the USPTO. They are expensive procedures. In this case you are concerned about a difference between a product and its description in a patent. That would not be grounds for challenging the patent. A patent owner has no obligation to make a product that matches any description or definition in their patent. Also, the maker of a product does not need to have a patent at all, let along one that matches a manufactured product.
If the product has a patent number printed on it, that is another issue. There is a law against mis-marking of products with a non-applicable or expired patent. However the America Invents Act ("AIA") took the teeth out of it.